1. Field of the Invention
The present invention relates to measuring equipment for providing accurate measurements of a plurality of properties of a sample, such as dissolved oxygen, conductivity, and pH and more particularly to measuring equipment having a plurality of probes that can accurately measure a characteristic of a sample while the other probes are isolated from any interference in the measurement signals.
2. Description of Related Art
Various forms of measuring equipment for measuring at least two properties of a sample, such as at least two of dissolved oxygen, conductivity, and pH, wherein the appropriate probes are emerged in the same sample are known. Problems that are experienced in such measuring equipment are that there is an interference and an influence in the measurement by one of the probes as a result of the immersion of the other probes in the same sample. Examples of this type of measuring equipment are shown in FIGS. 4-6.
Referring to FIG. 4, one example of a configuration of conventional measuring equipment for measuring dissolved oxygen and pH is shown. The dissolved oxygen probe 31 has a pair of signal cables 32, 33 connected to the probe. One signal cable 33 has a reference potential or source connected. The other signal cable 32 is in turn connected to a current voltage converter circuit 34 whose output signal is connected to an analog to digital, A/D, converter circuit 35, which in turn is connected by a photo-coupler 36 to a processor and display unit 37. A power supply 38 is provided for driving the current-voltage converter circuit 34, the A/D converter circuit 35, and the photo-coupler 36, respectively.
A pH probe 39 is connected to a reference source by a reference line 41. In addition, a signal cable 40 is connected to an impedance converter circuit 42 whose output signal is connected to an A/D converter circuit 43. A photo-coupler circuit 44 can provide the output of the AID converter circuit 43 to the processor and display unit 37.
A power supply 45 can drive the impedance converter circuit 42, the A/D converter circuit 43, and the photo-coupler 44, respectively. In FIG. 4, the circuit I is capable of providing a measurement of the dissolved oxygen when the dissolved oxygen probe 31 is activated so that a signal is provided through the signal cable 32 to the current-voltage converter circuit 34 which is then connected to the A/D converter 35 to provide a digital output representative of the dissolved oxygen which is then connected to the processor and display unit 37 by the photo-coupler 36. The circuit II in FIG. 4 is used for measuring pH and comprises the pH probe 39, a signal cable 40, an impedance converter 42, A/D converter circuit 43, photo-coupler 44, and the processor display unit 37. The purpose of the photo-couplers 36 and 44 are to permit an isolation of the respective pH probe 39 and dissolved oxygen probe 41.
As can be appreciated, however, from reviewing the equipment configuration shown in FIG. 4, there is a duplication of circuit components since the A/D converter circuits 35 and 43, power supplies 38 and 45, and photo-couplers 36 and 44 are used, resulting not only in an increased number of component parts so that it can respectively fail, but also an increase in cost, as well as an increase in circuit area. These issues remain the same even when the dissolved oxygen probe 31 and the current voltage converter circuit 34 are replaced with a conductive probe and conductive voltage converter circuit.
FIG. 5 discloses an alternative arrangement of measuring equipment for measuring both dissolved oxygen and pH of a sample. In this configuration, switches 46 and 47 are installed, respectively, in the cable signal lines 33 and 41. When switch 47 is turned off (open condition), the pH probe 39 is isolated during the measurement of dissolved oxygen. Alternatively, switch 46 is turned off (open condition) to isolate the dissolved oxygen probe 31 when the pH measurement is being conducted. As can be seen, the current-voltage converter circuit 34 and the impedance converter circuit 42 can be both powered by the same power supply 49, while a common A/D converter circuit 48 is used, which is also powered by the same power supply 49. This design approach attempts to eliminate any interference or influence between the pH probe 39 in the dissolved oxygen measurement and any interference or influence of the dissolved oxygen probe 31 during a pH measurement. This design approach has another advantage in that only one A/D converter circuit 48 and power supply is required.
Problems and the potential for problems during the measurement cycle still exist since, when switch 46 is brought to an open state to measure pH, it takes a period of time to stabilize the potential at the dissolved oxygen probe 31 when the switch 47 is being opened and the switch 46 is being closed to measure the dissolved oxygen for the next cycle. Thus, the measurement cycle time is increased.
An alternative conductivity meter is shown in FIG. 6 and incorporates a switch 56 that is intermediately installed in the signal line cable 53 that connects the conductivity probe 51 to a reference source. Opening the switch 56 will isolate the conductivity probe 51 during a pH measurement. As can be seen, a single power supply 49 can be used to drive the conductivity-voltage converter circuit 54, the impedance converter circuit 42, and the respective A/D converter circuits 43 and 55. As a result of this design, it is possible to minimize interference and influences during the pH measurement cycle. However, when the switch 56 is electrically controlled, the switching by a semiconductor (for example, an analog switch) is used in place of a relay switch operated by electromagnetic in order to reduce the power consumption. When the switch 56 is turned on to measure conductivity, the ON resistance can exert an influence that may provide errors in the measurement results attained during the conductivity measurement. These errors have to be dealt with by a signal filtering procedure at the processor and display unit 37 and thereby increases or complicates the processing of the output signal.
The pH measuring and meter equipment field is subject to competitive pressures on the cost of equipment and accordingly there is still a demand for providing relatively simplified equipment at economical cost.